Maple Syrup Mould

Maple syrup mould. ©CooksInfo / 2019

Mould on the surface of maple syrup is a common household problem once the seal on syrup containers has been broken and the containers stored for a while.

The advice from food safety professionals is to discard the syrup, and store future syrup purchases in ways that way retard or avoid the mould development (and the financial loss!)

Straining the mould out will not remove any enzymes that the mould has added to the syrup, nor will heating it deactivate any toxins.

See also: Maple syrup, Maple Syrup: Canning or Freezing

Finding mould upon purchase

It is rare that you will find maple syrup to have gone mouldy upon freshly opening a sealed container of it from the store or producer.

Most maple syrup sold to consumers is hot packed now in order to prevent moulds developing in the containers. The hot packing involves putting boiled syrup straight into the containers before it has a chance to cool, then sealing the containers and tipping them so that — in theory — the hot syrup kills any mould spores in the air that entered the container as the cap was being put on.

Still, it’s not a fool-proof method. Sometimes the syrup isn’t hot enough, and sometimes the container cools down the syrup immediately upon contact just enough to lower the temperature below the necessary kill temperature for the spores. And, sometimes there are present some heat-resistant spores for whom a boiling temperature is just like a nice sauna:

“Researchers have identified several heat-resistant microbial species that can survive boiling temperatures for prolonged periods.” ^[1]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at
https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

Additionally, even if they were all destroyed during boiling, “spoilage organisms (spores) can re-enter into maple syrup at several access points during post-boil handling…” ^[2]Ibid.

If you break the seal on a container of maple syrup previously unopened and find mould, return it.

“If the syrup was moldy when they first opened it they should contact the producer and get a replacement.” ^[3]Isselhardt, Mark. University of Vermont Extension Maple Specialist. 13 March 2019. Accessed March 2021 at https://ask.extension.org/questions/546494

In fact, you’re doing the processor a favour, by letting them know of potential flaws in their packing process. One producer advises, “Please make note of the lot# on your bottle, the date and location of purchase, and send this information to [us].” ^[4]Cosman & Webb. FAQ. Accessed March 2021 at https://www.coswebb.ca/faq

Why mould should be rare in maple syrup

In theory, mould should be rare in maple syrup because maple syrup has what is called a “low water activity.”

Water activity is a measure of how much water is available to support the growth of microbes, such as bacteria, or in this case, moulds.

Tom Ross, Associate Professor in Food Microbiology, Food Safety Centre, School of Land and Food, University of Tasmania, says: “The water activity scale runs from 0 to 1. A water activity of 0 means completely dry, while a water activity of 1 is pure water, with nothing dissolved in it. Any food with a water activity less than 0.6 will prevent any microbial growth.” ^[5]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

The higher the concentration of sugar in a mixture, the more water molecules in the mixture are bound to the sucrose molecules in the sugar. This leaves less water free for the use of microbial cells. Maple syrup is quite high in sucrose, about 64%.

Estimates of the water activity ranges of maple syrup vary; you may see a range of numbers given from 0.84 to 0.88. ^[6] 0.841 to 0.855. Maple Syrup of Quebec. Fédération des producteurs acéricoles du Québec. Industry Sheet. April 2018. Pp 1-2. ^[7]”The water activity of maple syrup “is generally about 0.87 to 0.88” Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

The exact water activity will vary from producer to producer, perhaps from batch to batch.

Still owing to its extremely high sugar content. even though maple syrup appears liquid, in terms of life forms seeking moisture from it, it’s actually relatively dry.

Additionally, through osmosis the sugar attracts the water out of most mould cells that land in it, killing them.

See also: https://www.healthycanning.com/water-activity/

Moulds that can survive in maple syrup

Note that even though maple syrup has a low water activity range of 0.84 to 0.88, it’s still far from, say, the stop-dead number of 0.6 cited above, “which means some moulds can grow on maple syrup.” ^[8]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

A certain class of funguses called “xerophiles” can grow even where water availability is low. Xerophile means “dry loving”.

Many different actual types of funguses fall into the xerophile class, even some penicillin moulds. ^[9]Hodge, Kathie. The fungus in my maple syrup. Cornell University. 20 March 2007. Accessed March 2021 at https://blog.mycology.cornell.edu/2007/03/20/the-fungus-in-my-maple-syrup

These will happily colonize a batch of maple syrup, given the right conditions.

“Preliminary research conducted by Dr. Seanna Annis and undergraduate students in her lab have identified several fungal genera (Penicillium, Aspergillus, Cladosporium and Wallemia) within or on the surface of contaminated syrup.” ^[10]Integrated Research/Extension Grants 2012–2013. Maine Agricultural Center. Page 1. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/wp-content/uploads/sites/403/2016/04/MAC-projects-2012-2013.pdf

Impact of maple syrup mould on quality and safety

Moulds secrete enzymes as they feed, which can give the syrup off-flavours.

Some people describe mould-infected syrup as having a flat taste, with some of the acidic flavour notes of the maple syrup gone. Others describe a sour edge or musty notes. The flavour impacts likely depend on the type and quantity of mould that was present.

In addition to flavour changes, depending on the variety of mould which colonized the maple syrup, toxic substances may be given off:

“Some of these fungi have the potential to produce mycotoxins, which may be a food safety concern.” ^[11]Integrated Research/Extension Grants 2012–2013. Maine Agricultural Center. Page 1. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/wp-content/uploads/sites/403/2016/04/MAC-projects-2012-2013.pdf

Some of these mycotoxins may include mycophenolic acid and penitrems which can cause nausea, tremors, seizures, breathing issues, internal bleeding, etc. Others are possibly carcinogenic. Tom Ross, Associate Professor in Food Microbiology, Food Safety Centre, School of Land and Food, University of Tasmania, says:

“Some of the moulds that can grow [on food], such as Aspergillus flavus, produce compounds that, after prolonged exposure, are linked to different forms of cancer.” ^[12]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

Some people will just plain have allergic reactions to various moulds. Some people are allergic to mould-ripened cheeses and penicillin, for instance.

“A low percentage of people can experience allergic reactions to specific mould spores.” ^[13]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

Can you just skim the mould out of maple syrup

Straining the mould out will not remove any enzymes that the mould has added to the syrup, nor will heating it deactivate any toxins released by the mould.

It will be impossible to get all the mould out. Strands called hyphae, the roots and branches of the mould as it were, that may be nearly invisible to the naked eye, will grow down from the floating clumps on top of the syrup. And the spores cannot be removed by filtration as they are too small.

“Fungi are made up of more than just the visible material floating on the surface. Various species can grow in correct density maple syrup.” ^[14]Isselhardt, Mark. Quality Control in the Sugarhouse. University of Vermont Extension. Supplement to June 2018 Maple Syrup Digest. North American Maple Syrup Council.”

You shouldn’t boil mouldy maple syrup either, in the vain hopes of killing anything bad in it:

“The toxins we are talking about can survive heating at 160° C (320° F),” says Ross. ^[15]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

“Maple syrup, kept at room temperature, is known to get contaminated with moulds (usually from the air after the bottle is opened) and it’s possible that some of those moulds produce carcinogens,” says Ross. “The carcinogens can then get distributed in the whole bottle of maple syrup because they are soluble.” A small dose of these carcinogens probably won’t lead to illness, but repeated exposure might. “Maple syrup that has visible mould growth might contain aflatoxins. It’s hard to be certain without actually identifying the mould that is growing, and that takes a lot of time and money.” ^[16]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

Many people of course report having fished it out, eaten it and survived. In fact probably most of us have. But in various web discussion forums, people can be seen anecdotally also popping up to say they felt mildly to very sick afterward.

“HI everybody! To all of you who are reticent about throwing out maple syrup contaminated with mold, let me tell you my very recent experience; in fact, so recent, I could be quite vivid in the recollection of last weekend, but for your sakes I’ll not go into detail! Let me just say that till now I was simply straining the precious syrup to remove unwanted guests that showed up to partake of my heavenly delight. But it seems this time a more vehement sort took revenge!! Sunday, I enjoyed my syrup on pancakes with no disagreeable taste, but less than an hour later I found myself slumped onto my sofa, feeling queasy and feverish. After a few unbearable minutes, thinking about what was happening to me, wondering what to do, I decided to eject the culprit. I stumbled across the living room and made my way to the toilet, tickled the back of my throat and… I’ll spare you and keep this rated G. Needless to say, I IMMEDIATELY felt so much better. I guess I’ll never eat anymore moldy syrup. Beware…” — Brigitte on November 11th, 2009. ^[17]User comment on: Hodge, Kathie. The fungus in my maple syrup. Cornell University. 20 March 2007. Accessed March 2021 at https://blog.mycology.cornell.edu/2007/03/20/the-fungus-in-my-maple-syrup/

Of course without investigation, there is no way of knowing whether such reported incidents were the result of something else they ate at the same time, or actually of the mould-contaminated maple syrup. Or, of knowing whether non-incidents were the result of luck of the draw working in those other people’s favour, or of not knowing how many incidents have gone unreported.

It all may well depend on what type of mould you have growing in your maple syrup — and you won’t know without access to a lab and a trained mycologist — as well as what sensitivities you might have to which moulds.

Mark Isselhardt from the University of Vermont says:

“Do not ‘skim and reheat’ moldy syrup…” ^[18] Isselhardt, Mark. Quality Control in the Sugarhouse. University of Vermont Extension. Supplement to June 2018 Maple Syrup Digest. North American Maple Syrup Council.” “Throw out the syrup… The recommendations used to be to reheat and skim but we don’t say that anymore; there are just too many potential problems that reboiling and filtering just can’t fix. If the syrup was moldy when they first opened it they should contact the producer and get a replacement” ^[19]Isselhardt, Mark. University of Vermont Extension Maple Specialist. 13 March 2019. Accessed March 2021 at https://ask.extension.org/questions/546494

Tom Ross, Associate Professor in Food Microbiology, Food Safety Centre, School of Land and Food, University of Tasmania, says:

“The best advice is to not eat maple syrup that has signs of mould growth.” ^[20]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

Barbara Bartle of the Ontario Ministry of Agriculture says:

“Some moulds produce mycotoxins that are very harmful. Mycotoxins are chemicals that seep under and around the mould. They remain in a food even after the mould is removed. Liquid and semi-solid foods, such as jam and maple syrup, should be discarded if mould is found.” ^[21]Bartle, Barbara, et al. Community Food Advisor, Resource Binder. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Section 6 : Food Storage and Preservation. Page 1. April 2013.

In food, the spread of mould goes far beyond what is visible. The Province of Manitoba Safe Food Guidelines say:

“Discard mouldy jams, syrups, nuts, or grains. Mould spreads in these products and can be dangerous.” ^[22]Grade 12 active healthy lifestyles. Manitoba Education, Citizenship and Youth. Winnipeg, Manitoba, Canada. 2009. RM 10–NU: Safe Food Guidelines, page 311.

Ohio State University Extension says:

“Do not simply remove the mold and reheat the product. Some microorganisms produce toxins as they grow, and these toxins could make you sick. The product should be discarded.” ^[23]Drake, Barbara H. Selecting, Storing, and Serving Ohio Maple Syrup. Ohio State University Extension. HYG-5522. 25 February 2010. Accessed March 2021 at https://ohioline.osu.edu/factsheet/HYG-5522

Outdated advice

The advice used to be to scrape off and discard the mould, then use the syrup. Some people later added a step to that advice so that it read: scrape off and discard mould, then boil the syrup, then use.

The maple processing industry made some assumptions in the past that are now being disproved by both science and experience:

“Previously, the maple industry assumed that… if the syrup was contaminated, boiling the syrup for 15 minutes would kill any contamination and threat to human health.” ^[24] Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. page 1. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/

What also needs to be acknowledged is that you will still see old, dated advice from otherwise reputable sources floating around. Be aware that such advice is dated — for instance, the advice immediately below dates from 1998 — and the science has moved on. We have learned more about the food safety issues involved since it was given:

“[SAMPLE OF OUTDATED ADVICE] Should mold form on syrup that has been stored for several months, simply bring the syrup to near boiling (190° F), remove the mold by skimming, and repack the syrup in a clean container.” ^[25]Davenport, Anni L. and Lewis J. Staats. Maple Syrup Production for the Beginner. Cornell and PennState Extension. 1998. Accessed March 2021 at https://extension.psu.edu/maple-syrup-production-for-the-beginner

Whose advice will you follow

Whose mould safety advice should you follow, that from processors, or that from food safety professionals?

Beef and chicken farmers are certainly experts in raising their respective livestock, but doctors and food safety experts know a heck of a lot more about E.coli and salmonella. Similarly, maple syrup processors know a lot about producing top quality maple syrup, but whose advice about moulds is worth more — theirs, or experts in mycology and food safety?

The question of the safety of mould — what effect its compounds can have once inside your body — arguably goes way beyond the field of expertise that maple syrup producers have certification in, and goes into another field entirely.

Storing to prevent mould

Unopened metal, tin, glass or squeeze-type plastic containers of maple syrup will have a long shelf life at normal room temperatures in the cupboard owing to both the high sugar content of the syrup, and to the container having been hot packed at 85° C / 185° F, and then immediately sealed. The Quebec Maple Syrup Producers say that as far as cans go, “an unopened can of maple syrup will keep for several years.” ^[26]Quebec Maple Syrup Producers (QMSP) Association. Keeping your maple syrup. Accessed February 2021 at https://maplefromcanada.ca/products/maple-syrup/

Beige plastic maple syrup jug with limited storage life. Kiliweb / Open Food Facts / CC BY-SA 3.0

The grey / beige plastic containers are different, though: they are porous and will slowly let oxygen through, which can shorten the storage life of the syrup, and should any mould spores have survived the processing, allow them to grow. Containers made out of such material should be refrigerated as soon as brought home, even if unopened.

“Container types can impact mould growth in maple syrup. The traditional thick walled, high density polyethylene (HDPE) plastic jugs used to store maple syrup are somewhat permeable to oxygen, which allows mould to grow. Some manufacturers coat the exterior surface with a polyvinylidene chloride copolymer (“XL” coating) which serves as an oxygen barrier. Using a bottle with this coating can help limit mould growth after packing.” ^[27]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

Always tightly seal and refrigerate all maple syrup containers after opening them, regardless of the material they are made of.

Refrigeration will slow the growth of xerophile moulds in your maple syrup. The moulds need food, some water, oxygen and warmth to grow.

For best long-term quality, freeze the syrup.

See: Maple Syrup, ‘canning’ or freezing?

Ferment

A similar issue that can occur is maple syrup fermenting, but this should only occur at a consumer level if the syrup was improperly processed by the producer, or if the syrup has been adulterated, say, with water.

“Fermented syrup usually develops from one of two problems with the product. If syrup has not been boiled enough to concentrate the correct amount of sugar, then the syrup may work like apple cider. At times, we find correct density syrup fermented and that is usually from syrup stored in barrels that have not been properly cleaned. Even barrels that have been previously steam cleaned may have moisture in them that have revealed yeast, mold, and bacteria in great numbers. Syrup that is fermented will have a sickening sweet flavor, at times a honey like similarity. Depending on the type of ferment, it may have an alcoholic or fruity taste. Severe ferment may have a foamy appearance.” ^[28]Marckres, Henry J. Maple Syrup Off-Flavors. Vermont Agency of Agriculture Food and Markets. Undated. Accessed March 2021 at https://extension.umaine.edu/maple-grading-school/wp-content/uploads/sites/47/2020/07/Off-flavors-opt.pdf

It is not clear what the food safety implications of this fermentation are, but you will probably wish to discard anyway based on food quality reasons.

Sources

Hodge, Kathie. The fungus in my maple syrup. Cornell University. 20 March 2007. Accessed March 2021 at https://blog.mycology.cornell.edu/2007/03/20/the-fungus-in-my-maple-syrup/

Leuty, Todd. Preventing mould in packed maple syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 27 March 2014. Accessed March 2021 at http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2014/04hrt14a2.htm

Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup